The fuel and lubrication oil used in the internal combustion engine contains sulfur, therefore the exhaust gas contains SOX. However, this SOX acts to greatly lower the performance or durability of the exhaust gas purification catalyst or other post-treatment device arranged in an engine exhaust passage, therefore the SOX in the exhaust gas is preferably removed.
Therefore, there is known an internal combustion engine in which an SOX trap catalyst able to trap the SOX contained in the exhaust gas is arranged in an engine exhaust passage (see Japanese Patent Publication (A) No. 2005-133610). This SOX trap catalyst has the property of trapping the SOX contained in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the SOX trap catalyst is lean, allowing the trapped SOX to gradually diffuse inside the NOX trap catalyst when the temperature of the SOX trap catalyst rises when the air-fuel ratio of the exhaust gas is lean, and as a result allowing the SOX trap rate to be restored. Therefore, this internal combustion engine is provided with an estimating means for estimating the SOX trap rate by the SOX trap catalyst. When the SOX trap rate falls below a predetermined rate, the temperature of the SOX trap catalyst is raised under a lean air-fuel ratio of the exhaust gas to thereby restore the SOX trap rate.
However, the inventors engaged in repeated research on this type of SOX trap catalyst. As a result, they discovered a new method enabling restoration of the SOX trap rate and learned that if using this method, it is possible to restore the SOX trap rate even better.